# Full text of "Text Book Of Mechanical Engineering"

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938

Appendix IIL

A  =  area of ashpit opening in square feet.

V  =  velocity of air through ashpit in feet per min.

tz   =  temperature of flue gases, found by pyrometer.

/!   =  temperature of stokehole.

rj  =  absolute temp, of do.

P   =  pressure of atmosphere in Ibs. per sq. ft.

w  =  weight of i cub. ft. of air at P in Ib. units.

k   =  coal used per m. in Ibs.

Then, AV cub. ft. of air per m. are used, whose weight per
cub, ft. (see p. 590)

Pxi         147x144       39-4

I ;f

r  V

1:1
ill

But —T—  = weight of air per Ib. of coal.

And, as          Total wt. x sp. ht. x temp, rise = total heat

Heat units               \   _   / AW#       \ p ,   _

' '    wasted per Ib. of coal /   ~  \    ^          /    P^S~ i)

Note that Cp = specific heat of the flue gases at constant
pressure and is almost always exactly '25.

The second method may be briefly stated as follows:—

(a)  Find % volumetric composition of flue gases, and con-

vert at once into % weight composition,
(i cub. ft. CO2 at 32° weighs '122 Ib
i cub. ft. N at 32° weighs '078 Ib.
i cub. ft O at 32° weighs '089 Ib.)

(b)  Find weight of CO2 per Ib. of coal, thus :

. °/°^, (i Ib. C + 2-66 Ib. 0) = • °'° Ct (3:66 Ib, CO2)

(c)  Hence find by proportion the total weight of each flue

gas separately, per Ib. of coal.

(d)  From

Cp x wt. of each gas per Ib coal x rise of temp.

the heat given to each gas may be found, and thus the
total heat given to the mixture can be deduced per Ib.